Load shedding is commonly employed by electric utilities during periods (e.g., hot summer days) when the amount of power demanded from the electric utility is extraordinarily high. Typically, electric utility companies offer monetary incentives to certain high demand customers, such as factories and office buildings, in order to allow the electric utility to reduce the amount of power delivered to those customers during periods of high power demand.
Fluorescent lighting accounts for a significant portion of the total power that is demanded from an electric utility. Accordingly, fluorescent lighting systems that accommodate load shedding by dimming the lamps (thus reducing the amount of power) in response to a load shed command are very desirable.
Fluorescent lamps require ballasts that provide a high voltage for igniting the lamps, as well as a magnitude-limited current for operating the lamps at an appropriate power level. As compared with conventional “core and coil” magnetic ballasts, electronic ballasts are known to provide enhanced energy efficiency and other benefits (e.g., negligible visible flicker). However, electronic ballasts are more difficult to control than magnetic ballasts, especially in dimming applications.
Electronic dimming ballasts are well known in the art. Dimming ballasts typically include a high frequency inverter and complex circuitry for precisely controlling (e.g., via frequency or duty cycle control) the amount of power that the inverter delivers to the lamps. Additionally, dimming ballasts typically require dedicated low voltage wiring for receiving an input from a special dimming controller. As a result, electronic dimming ballasts are generally much more expensive (in terms of both material and installation costs) than ordinary fixed light output electronic ballasts. Consequently, dimming ballasts account for but a small fraction of the electronic ballasts that are currently in use.
What is needed, therefore, is an electronic ballast that includes economical load shed circuitry for reducing power consumption in response to a load shed command from an electric utility. Such a ballast would represent a significant advance over the prior art.